Metal foil for capacitor, solid electrolytic capactor using the foil and production methods of the foil and the capacitor

ABSTRACT

A metal foil for capacitor element is produced through a process comprising steps of etching and then electrochemically forming a metal foil after making cut lines each in a shape of a capacitor element with at least a part of a portion predetermined to be an anode-leading-out-part left uncut. The step of etching the foil is preferably performed with the anode-leading-out-parts being protected by protective material. Solid electrolytic capacitor elements prepared by using the metal foil have narrow variation in capacitance.

CROSS-REFERENCE TO RELATED APPLICATION

This is an application based on the prescription of 35 U.S.C. Section111(a) with claiming the benefit of filing date of U.S. Provisionalapplication Ser. No. 60/407,974 filed Sep. 5, 2002 under the provisionof 35 U.S.C. Section 111(b), pursuant to 35 U.S.C. Section 119(e)(1).

TECHNICAL FIELD

The present invention relates to a method for producing a metal foil forcapacitors used in various electronic instruments, and a capacitorprepared by using the foil. More specifically, the present inventionrelates to a method for etching and electrochemically forming a metalfoil for multilayer solid electrolytic capacitors, and a solidelectrolytic capacitor using a metal foil obtained by the method.

BACKGROUND ART

Developments of chip-type or small-size electronic components areaggressively proceeding to cope with the requirement for downsizing ofelectronic instruments, high-density packaging of print substrates,promotion of packaging efficiency and the like. Along with thedevelopments, requirement for production of chip-type or small-sizeelectrolytic capacitors used as components is increasing. In this pointand also in view of easy handleability, development and dissemination ofsolid electrolytic capacitors not using an electrolytic solution areabruptly growing in recent years.

Generally, a chip-type solid electrolytic capacitor is composed byforming an oxide dielectric film on an etched valve-acting metal foiland thereon forming cut-out grooves each in the element form (see,JP-A-5-283304 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application), or by a process where afterfixing foils each cut out into an element shape on a metal-made supportand thereon forming a solid electrolyte, a cathode electricallyconducting layer comprising carbon paste and silver paste is formedthereon and an outer jacket part enclosing the whole is formed.

Among valve-acting metals such as aluminum, tantalum, niobium andtitanium, aluminum is advantageous in that the surface area can beeasily enlarged by an etching treatment and the oxide film formed on thesurface by anodization (electrochemical formation) using the aluminum asthe anode can be used as a dielectric material, therefore, a smallcapacitor having a large capacitance can be produced at a low cost ascompared with other capacitors. By virtue of these properties, analuminum solid electrolytic capacitor particularly for low voltage useis widely used.

Presently, the electrode foil for use in the aluminum solid electrolyticcapacitor is an aluminum foil which is electrochemically or chemicallyetched to enlarge the surface area and then subjected to punching intothe shape of a product pattern and electrochemical formation of the cutend part.

Methods for etching an aluminum foil include a DC (direct current)electrolytic etching method where an aluminum foil is etched in anelectrolytic solution comprising a chloride ion-containing aqueoussolution having added thereto a phosphoric acid, a sulfuric acid, anitric acid or the like by passing a DC current using the aluminum foilas the positive electrode and an electrode disposed adjacently to thealuminum foil as the negative electrode, and an AC (alternating current)electrolytic etching method where an aluminum foil is etched in anelectrolytic solution comprising a chloride ion-containing aqueoussolution having added thereto a phosphoric acid, a sulfuric acid, anitric acid or the like by passing an AC current between electrodesdisposed at both sides of the aluminum foil (indirect supply ofelectricity) or between the aluminum foil and each of the electrodesdisposed at both sides thereof (direct supply of electricity).

In the DC electrolytic etching, the etching proceeds while formingtunnel-like pits in crystallographic orientation. On the other hand, inthe AC current electrolytic etching, the etching proceeds while formingetching pits sequentially connected like a rosary in random directionsand this is advantageous for enlarging the surface area (areaenlargement). Therefore, AC electrolytic etching is predominantlyperformed for the etching of an aluminum foil, however, a method ofcombining these two methods and a method of gradually increasing the ACvoltage have been also proposed (see, JP-A-11-307400). In addition, amethod involving adjustments of the waveform, amplitude and the like ofthe AC to improve the effective area enlargement (JP-A-7-235456) and amethod where aluminum comprising a specific metal which works as astarting point of etching corrosion is used (JP-A-7-169657) have beenalso proposed.

After a valve-acting metal foil is formed into a porous valve-actingmetal foil by electrochemical etching or after a dielectric layer isformed thereon, when the foil is cut into a capacitor element shape,cracks are generated in the porous layer formed by etching in thevicinity of cut face, and burrs are generated in the cut end part torender the part rough.

These cracks, burrs and the like on the cut edge surface generated atthe time of cutting give rise to deterioration of capacitor properties.

In a step of attaching electrically conducting polymer to the foil toform a cathode part, masking is applied to the boundary between theanode-leading-out-part and the cathode part for the purpose ofpreventing the treating solution from creeping up to theanode-leading-out-part. However, electrically conducting polymer easilyspreads beyond the masking material toward the anode part, which resultsin increase of leakage current.

WO 02/063645 has proposed a method where an etching layer is formed onthe cut edge surface of a foil cut out into a capacitor element shape byelectrolytic etching and at this time, burrs on the cut edge part aredissolved. However, in this method, etching is likely to be localized onthe cut end part of the foil and this makes it difficult to control thecurrent distribution, and another problem is involved that the cut edgepart is dissolved or the strength of the part is decreased so quicklythat effective area of the element decreases, failing in achieving amass-production process of etched foils having a stable quality.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a chemically formedfoil for producing capacitors having a uniform shape with narrowvariation in capacitance, and a production method of the foil.

Another object of the present invention is to provide capacitor elementsusing the foil and a production method thereof.

The present inventors have completed the present invention based on thefindings that production of metal foils for capacitors with a narrowvariation in capacitance can be achieved by a process involving thesteps: making a number of cut lines having a predetermined fine width invalve-acting metal foil material such that each cut portion has a shapeof an anode for a capacitor element for the purpose of producingcapacitor elements in quantity at a time; etching the surface of themetal foil and the cut edge surface; and performing electrochemicalformation.

Also, the present inventors have found that etching layers can be formedonly on the portions to be cathode parts by performing etching afterprotecting the portions to be anode-leading-out-parts of capacitorelements with a protective material, so that, in a subsequent step ofattaching electrically conductive polymer to the cathode parts, theanode-leading-out-parts can be satisfactorily prevented from thetreating solution creeping up by masking, and as a result, production ofa capacitor having properties of stable capacitance and reduced leakagecurrent can be achieved.

More specifically, the present invention relates to the followingvalve-acting metal foil for capacitor, solid electrolytic capacitorusing the foil and production methods of the foil and the capacitor:

-   -   1) a method for producing a metal foil for capacitors,        comprising a step of making cut lines in a valve-acting metal in        a shape of a capacitor element with at least a part of a portion        predetermined to be an anode-leading-out-part left uncut, a step        of etching the cut edge surface generated in the previous step        and the surface part of the valve-acting metal foil, and a step        of electrochemically forming the metal foil;    -   2) the method for producing a metal foil for capacitors as        described in 1 above, wherein the etching is performed after        protecting the portion predetermined to be the        anode-leading-out-part of a capacitor element with a protective        material;    -   3) the method for producing a metal foil for capacitors as        described in 2 above, wherein the protective material is removed        after etching, and then the step of electrochemically forming is        performed;    -   4) the method for producing a metal foil for capacitors as        described in 2 above, wherein the protective material is removed        after electrochemically forming the etched foil;    -   5) the method for producing a metal foil for capacitors as        described in 2 above, wherein the protective material is removed        after etching, and masking is applied to the boundary between        the anode-leading-out-part and the region to have a solid        electrolytic layer formed thereon as a cathode part before        performing the step of electrochemically forming the region to        be a cathode part;    -   6) the method for producing a metal foil for capacitors as        described in any one of 1 to 3 above, wherein each of the cut        portions has a quadrangular-shape having uncut portion,        U-shape(horseshoe-shape) or semicircular shape;    -   7) the method for producing a metal foil for capacitors as        described in 1 above, wherein the cut edge surface has an acute        interior angle A of 30° or more with respect to the metal foil        surface;    -   8) the method for producing a metal foil for capacitors as        described in 1 above, wherein the width d of the cut line is        twice or less the thickness of the metal foil;    -   9) the method for producing a metal foil for capacitors as        described in 1 above, wherein a plurality of metal foils for        capacitors is produced in a single batch process by making a        plurality of cut lines each having a shape of a capacitor        element in a single valve-acting metal foil;    -   10) the method for producing a metal foil for capacitors as        described in 1 above, wherein the foil consists of at least one        valve-acting metal selected from a group of aluminum, niobium        and tantalum;    -   11) the method for producing a metal foil for capacitors as        described in 1 above, wherein the valve-acting metal foil has a        thickness of 0.05 to 1 mm;    -   12) the method for producing a metal foil for capacitors as        described in 1 above, wherein the valve-acting metal foil is an        aluminum foil containing at least one element selected from the        group consisting of Si, Fe, Cu, Zn, Ni, Mn, Ti, Pb, B, P, V and        Zr;    -   13) the method for producing a metal foil for capacitors as        described in 12 above, wherein the total content of the elements        other than aluminum contained in the foil is from 1 to 1,000 ppm        by mass in terms of atom;    -   14) the method for producing a metal foil for capacitors as        described in 12 above, wherein the aluminum foil contains Si in        an amount from 1 to 100 ppm by mass, Fe in an amount of from 1        to 100 ppm by mass and Cu in an amount of from 1 to 100 ppm by        mass;    -   15) the method for producing a metal foil for capacitors as        described in 1 above, wherein the etching is AC electrolytic        etching using at least one waveform selected from the group        consisting of sine wave, rectangular wave and triangular wave;    -   16) the method for producing a metal foil for capacitors as        described in 1 above, wherein the etching is AC electrolytic        etching where terminals are provided on the valve-acting metal        and on electrodes placed to both sides of the valve-acting metal        and AC current is directly supplied to the terminal provided on        the valve-acting metal;    -   17) the method for producing a metal foil for capacitors as        described in 1 above, wherein the etching is DC electrolytic        etching;    -   18) a metal foil for capacitors, obtained by the production        method described in any one of 1 to 17 above;    -   19) the metal foil for capacitors as described in 18 above,        wherein the edge of the portion to be a cathode has a curvature        radius r of 0.1 to 500 μm;    -   20) the metal foil for capacitors as described in 18 above,        comprising, on the surface of the metal foil and the cut edge        surface, porous layers formed on a portion where solid        electrolyte is to be formed, wherein the thickness of the porous        layer on the cut edge surface, T2, is more than 1 μm, and has a        following relationship with the thickness of the porous layer on        the surface of the metal foil, T1:        T 2/T 1≦2;    -   21) a solid electrolytic capacitor element, comprising a solid        electrolyte layer and an electrically conducting layer in the        order on the metal foil as described in any one of 18 to 20        above;    -   22) the solid electrolytic capacitor element as described in 21        above, wherein the solid electrolyte layer comprises an        electrically conducting polymer;    -   23) the solid electrolytic capacitor element as described in 22        above, wherein a monomer of forming the electrically conducting        polymer is a monomer compound containing a heterocyclic        5-membered ring or a monomer compound having an aniline        skeleton;    -   24) the solid electrolytic capacitor element as described in 23        above, wherein the monomer compound containing a heterocyclic        5-membered ring is a compound selected from the group consisting        of pyrrole, thiophene, furan, polycyclic sulfide and        substitution derivatives thereof;    -   25) the solid electrolytic capacitor element as described in 23        above, wherein the monomer compound containing a heterocyclic        5-membered ring is a compound represented by the following        formula (I):        wherein the substituents R¹ and R² each independently represents        a monovalent group selected from the group consisting of a        hydrogen atom, a linear or branched, saturated or unsaturated        hydrocarbon group having a carbon number of 1 to 10, an alkoxy        group, an alkyl ester group, a halogen, a nitro group, a cyano        group, a primary, secondary or tertiary amino group, a CF₃        group, a phenyl group and a substituted phenyl group, the        hydrocarbon chains of R¹ and R² may combine with each other at        an arbitrary position to form a divalent chain for forming at        least one 3-, 4-, 5-, 6- or 7-membered saturated or unsaturated        hydrocarbon ring structure together with the carbon atoms        substituted by the groups R¹ and R², and the combined ring chain        may arbitrarily contain a bond of carbonyl, ether, ester, amide,        sulfide, sulfinyl, sulfonyl or imino;    -   26) the solid electrolytic capacitor element as described in 23        above, wherein the monomer compound containing a heterocyclic        5-membered ring is a compound selected from        3,4-ethylenedioxythiophene and 1,3-dihydroisothianaphthene;    -   27) a multilayer solid electrolytic capacitor obtained by        stacking a plurality of capacitor elements as described in 21        above;    -   28) a method for producing solid electrolytic capacitor        elements, comprising a step of making cut lines each having a        shape of a capacitor element with at least a part of a portion        predetermined to be anode-leading-out-part left uncut in a        valve-acting metal foil, a step of etching the cut edge surface        generated in the previous step and the surface of the        valve-acting metal foil, a step of electrochemically forming the        etched metal foil to form an oxide dielectric film after cutting        the foil into stripes each having a comb-like shape where foil        portions each cut in a shape of an element link together in        anode-leading-out-parts, a step of forming a solid electrolyte        layer on the oxide dielectric film layer, a step of forming an        electrically conducting layer on the solid electrolyte layer,        and a step of severing the foil pieces each in a shape of a        capacitor element by making a cut in the anode-leading-out-part        of each piece.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for explaining the process of producing themetal foil for capacitors of the present invention.

FIG. 1(A) is a state diagram showing a valve-acting metal foil having aplurality of cut lines each having a shape of a capacitor element forthe purpose of fabricating a plurality of capacitor elements from thesingle metal foil.

FIG. 1(B) is a state diagram showing the valve-acting metal foil of FIG.1(A) with the portions predetermined to be anode-leading-out-parts ofcapacitor elements being protected with protective material.

FIG. 1(C) is a state diagram showing the valve-acting metal foil of FIG.1(B) with the protective material being removed from portions to beanode-leading-out-parts of capacitor elements after etching the wholefoil.

FIG. 1(D) is a state diagram showing the portions to be cathodes ofcapacitor elements being electrochemically formed after cutting out ofthe valve-acting metal foil of FIG. 1(C) in a shape of a comb.

FIG. 2 is an enlarged view of FIG. 1(A) showing a cut made in a shape ofa capacitor element in a valve-acting metal foil.

FIG. 3 is a cross-sectional view of x-x of FIG. 2.

FIG. 4 is a cross-sectional view of a solid electrolytic capacitorelement of the present invention.

FIG. 5 is a cross-sectional view of one embodiment of a multilayer solidelectrolytic capacitor fabricated from the solid electrolytic capacitorelements of the present invention.

DETAILED DESCRIPTION OF INVENTION

The methods of the present invention are described below.

(1) Valve-Acting Metal

The valve-acting metal foil for use in the present invention is a metalfoil having a valve action, such as aluminum, niobium, tantalum,aluminum alloy, niobium alloy and tantalum alloy. The metal used in thepresent invention may be in form of a plate as well as a foil. Preferredexamples thereof include a foil of aluminum or aluminum alloy, which iscommercially available as a roll or a plate. The thickness may besufficient if it is in a range for enough strength of the aluminum foilto be ensured after etching. The thickness is, for example, from 0.05 to1 mm, preferably from 0.08 to 0.4 mm, more preferably from 0.1 to 0.2mm.

The aluminum may contain at least one element selected from the groupconsisting of Si, Fe, Cu, Zn, Ni, Mn, Ti, Pb, B, P, V and Zr, andpreferably the aluminum contains each of such an element in an amount of1 to 100 ppm by mass, more preferably from 10 to 50 ppm by mass, basedon the entire amount of aluminum foil, provided that the total amount ofthese elements is in a range of 1 to 1,000 ppm by mass.

In particular, an aluminum containing Si in an amount of 1 to 100 ppm bymass, Fe in an amount of 1 to 100 ppm by mass and Cu in an amount of 1to 100 ppm by mass is preferred, and an aluminum containing Si in anamount of 10 to 50 ppm by mass, Fe in an amount of 10 to 50 ppm by massand Cu in an amount of 10 to 50 ppm by mass is more preferred.

Examples of the aluminum alloy mainly consisting of aluminum includealloys of aluminum with one or more members of silicon, titanium,zirconium, tantalum, niobium and hafnium.

The size of the original valve-acting metal foil to be made cuts in isnot limited, as long as it is large enough to fabricate a plurality of,for example, plate-like capacitor elements. Specifically, thevalve-acting metal foil preferably has a size large enough for aplurality of capacitor elements, as a plate-like element unit, eachhaving a width of 1 to 50 mm and a length of 1 to 50 mm, more preferablya width of 2 to 20 mm and a length of 2 to 20 mm, still more preferablya width of 2 to 5 mm and a length of 2 to 6 mm, to be taken out.

(2) Formation of Cut Lines

The step of making cut lines in the foil is described by referring todrawings.

In an embodiment as shown by FIG. 1(A), cut lines (cut grooves) 5 havinga predetermined line width and each forming a shape of a capacitorelement with at least a part of a portion to be ananode-leading-out-part left uncut, are formed for the purpose offabricating 30 total (in 10 rows×3 lines) from the single foil (plate).

As shown by FIG. 2 which is an enlarged view of FIG. 1(A), the cut line(cut groove) 5 is made, leaving uncut a portion 2 predetermined to bethe anode-leading-out-part or to take out anode electricity in the finalcapacitor element. In this embodiment, the cut line has a square shapewith one side open (uncut), but the cut line may have any shape such asquadrangular shape with its corner(s) being angular or rotundate, Ushape (horseshoe shape) and semicircular shape as long as it can form acapacitor element. In FIG. 2, only one cut line is shown but a pluralityof cut lines may be formed on the valve-acting metal foil at one time orby lots and the arrangement of the cut lines is not limited to the oneshown in FIG. 1(A), as long as the arrangement causes no problem in thelater steps.

FIG. 3 is a view schematically showing the cross section along the X-Xline in FIG. 2. The width d of the cut line (in the case where the widthdiffers between front and back surfaces of the foil, d represents thesmaller width) needs to be twice or less the thickness of the foil. Forexample, in electrolytic etching, since the dissolving amount of thevalve acting metal is mainly dominated by quantity of electricity andshapes of pores and grooves formed in the etching process are greatlyinfluenced by current density, it is important to adjust electriccurrent properly.

In the etching process using direct supply of electricity where aterminal is provided on the valve acting metal foil, the electriccurrent goes to and fro between the foil and the counter electrodes.Accordingly, provided that the foil and electrodes are parallel witheach other, the electric current flows in the vertical direction againstthe surfaces of the foil and the electrodes. However, in a case wherethe foil has a cut line, the electric current does not flow straightvertically but flows forward the cut edge or cut surface of the cutline. On the other hand, the current flows through a route to whichresistance becomes small. That is, the electricity which flows from aportion of an electrode near the cut line tends to go toward not theplain surface of the foil but the cut edge or cut surface of the cutline. Therefore, the larger the width of the cut line is, the moreelectricity converges on the cut edges to intensively etch the cutedges.

The surface area of the foil decreases by the width of the cut lineformed thereon, and increases by the cut surface area. That is, thenarrower the cut line, the less electricity converges on the cut edgesand cut surfaces of the cut line, thus preventing the cut edges frombeing excessively dissolved.

Specifically, assuming that the thickness of the foil is t and the widthof the cut line is d, if the cut edge surface is formed verticallyagainst the plane surface of the foil, the value of decrease in thewhole surface area of the foil is 2d and the value of increase is 2t.The larger “2d” the increase value “2t” is than the decrease value, themore likely the electric currents converge on the cut edges. For thepurpose of preventing excessive dissolution of the cut edge surface, itis preferable that the decrease value (2d) is twice or less the increasevalue (2t), that is, 2t×2≧2d. In other words, formation of the cut linehaving the width d which is twice or less the foil thickness tcontributes to alleviation of electricity conversion, thereby preventingexcessive dissolution of the cut edge surface.

Generally, the metal foil used in the present invention is 1 mm or less,preferably 0.4 mm or less, more preferably 0.2 mm or less. Accordingly,the width of the cut line is 2 mm or less, preferably 0.8 mm or less,more preferably 0.4 mm or less. If the width exceeds 2 mm, the electriccurrents converge on the cut edge surface and the cut edge surface islocally dissolved to cause decrease in the effective area of the elementand consequently reduction in the capacitance.

The cut line may be formed, for example, by cutting with a cutter,Thomson blade cutting, mold punching or laser cutting. The cut is madeon an angle such that either one of the front surface and the backsurfaces of the foil forms an obtuse interior angle with the cut surfacewhile the other surface forms an acute interior angle A with the cutsurface (in the embodiment shown in FIG. 3, the front surface of thefoil forms an obtuse interior angle with the cut surface while the backsurface forms an acute interior angle with the cut surface). In thepresent invention, the acute angle A is preferably 30° or more, morepreferably 50° or more. If the angle is less than 30°, etching proceedsfrom the sharp-edged portion and the potion is dissolved outexcessively, as a result, the effective area is decreased, which causesa wider variation in the capacitance.

(3) Etching

In etching the metal foil after cut lines are formed on the valve-actingmetal foil, the etching is performed by dipping the whole metal foil inan electrolytic solution prepared by adding a phosphoric acid, asulfuric acid, a nitric acid, an acetic acid, an oxalic acid or the liketo an aqueous solution containing chloride ion.

The electrolytic solution used for the etching is a solution containingat least chloride ion and thereto, at least one of a solution containingsulfate ion, phosphate ion, acetate ion, oxalate ion or the like and asolution additionally containing alkali metal ion or alkaline earthmetal ion may be added.

In an embodiment as shown in FIG. 1(B), a single valve-acting metal foilwhere thirty cut lines 5 each in a shape of a capacitor element are madein an arrangement of 3 lines×10 rows is subjected to etching treatment.It is preferable that portions 2 predetermined to beanode-leading-out-parts for capacitor elements are protected withprotective material 4 a. The protecting treatment with the protectivematerial may be applied to both front and back surfaces of the foil asneeded. This protecting treatment contributes to reduction in the ratioof defective products caused by bad electrical contact in fabrication ofa multilayer solid electrolytic capacitor composed by stacking pluralityof capacitor elements.

The protective material usable in the etching step may be any materialas long as it can be closely adhered to the valve-acting metal foil (forexample, aluminum foil) and can be stably present on the portions to beprotected without causing a reaction with the electrolytic solution(etching solution). Examples of the protective material include anacryl-base resin, a polyethylene sheet and a resist material. Squarebars of such a material may be placed to sandwich the pertinent portionand fixed with a pressure-sensitive adhesive tape, or such a materialmay be coated on that portion. The metal foil applied with thisprotective material is dipped in the electrolytic solution to etch andthen the protective material is removed as shown in FIG. 1(C).

The etching is preferably performed by AC etching under the conditionssuch that the frequency is from 1 to 1,000 Hz, the current density isfrom 0.025 to 4 A/cm² and the etching electricity is from 0.02 to 2,000C/cm². It is preferred to gradually increase the current density of theAC current and thereafter perform the AC electrolytic etching at aconstant electric current.

In the case of AC current, the current preferably has a waveformcontaining, for example, at least one of sine wave, triangular wave andrectangular wave.

Also, DC electrolytic etching and AC electrolytic etching may be used incombination by performing first DC. electrolytic etching and then ACelectrolytic etching. The etching may also be performed only by DCelectrolytic etching.

Whichever of the etching modes among AC, DC, or the combination thereofis employed, the current must be fed such that the valve-acting metalacts as a counter electrode to the electrodes placed to both sidesthereof.

It is preferred that etching is performed by an AC electrolytic etchingmethod where terminals are provided on the valve acting metal and onelectrodes placed to both sides of the valve-acting metal, and alternatecurrent is directly fed in between the valve-acting metal andelectrodes. According to this method, the cut surface can beappropriately etched as well.

After the electrolytic etching, water washing is performed to remove thecomponents of the electrolytic solution. Particularly, in order toreduce the remaining chloride ion, the water washing may be performedafter washing the metal foil with a nitric acid solution, a sodiumaluminate solution, an aluminum hydroxide solution or the like. Themetal foil may be further washed with a solution containing anelectrolytic solution for use in the formation of a dielectric film byanodization.

Also, chemical etching may be applied to enlarge the surface. In thechemical etching, a nitric acid, a ferric chloride or the like can beused.

In the metal foil thus obtained, the cut edge surface has a curvatureradius of 0.1 to 500 μm, preferably from 1 to 100 μm, more preferablyfrom 2 to 50 μm. If the curvature radius is less than 0.1 μm, the cutedge surface cannot exhibit an effect as a curved surface, failing inreducing leakage current.

The thickness T2 of the porous layer in the cut edge obtained by etchingis preferably twice or less the thickness T1 of the porous layer in theflat surface of the metal foil. If T2 exceeds twice T1, the etchinglayer in the cut surface decreases in the strength and cracking occursin the etching layer due to pressure generated at the time of stackingor sealing capacitor elements. FIG. 1(B) shows a case of applying aprotective material 4 a to the anode-leading-out-part 2, but in the casewhere protecting with a protective material is not applied, a porouslayer is formed also on the anode-leading-out-part 2 by etching.

(4) Electrochemical Formation

Subsequently, the protective material, if applied in the etchingtreatment, is removed off as shown in FIG. 1(C). Then, the etchedvalve-acting metal foil is cut into comb-like metal foil strips 10(comb-like aluminum foil strip) as shown in FIG. 1(D), and then thewhole strips or at least the regions 6 which each are predetermined tohave a solid electrolyte formed thereon to be a cathode part, i.e. theportions other than (and below) the portions to beanode-leading-out-parts 2 shown in FIG. 2, are subjected toelectrochemical formation. Alternatively, electrochemical formation maybe performed after removing the protective material and applying maskingto the boundary part between the anode-leading-out-part 2 and the regionto form a solid electrolyte thereon as a cathode part in a later step,or may be performed without removing the protective material.

The electrochemical formation can be performed by various methods, andthe conditions for performing the electrochemical formation are notparticularly limited. For example, the electrochemical formation may beperformed by using an electrolytic solution containing at least one ionsuch as oxalate, adipate, borate or phosphate under the conditions suchthat the electrolytic solution concentration is from 0.05 to 20% bymass, the temperature is from 20 to 90° C., the current density is from0.01 to 600 mA/cm², the voltage is a numerical value according to theelectrochemical forming voltage of the foil treated. The conditions arepreferably such that the electrolytic solution concentration is from 0.1to 15% by mass, the temperature is from 40 to 85° C., the currentdensity is from 0.05 to 100 mA/cm².

After the electrochemical formation, for example, a dipping treatment inphosphoric acid for improving the water resistance, or a heat treatmentfor strengthening the film, may be performed, if desired.

By the above treatment steps, the valve-acting metal foil of the presentinvention is obtained.

(5) Masking

Next, a solid electrolyte is formed to be a cathode part. If desired,masking 4 b is applied as a pretreatment. The masking has a function ofpreventing the treating solution from creeping up onto the masked partin the steps of forming an electrolyte layer and forming an electricallyconducting layer, thereby completely insulating the electricallyconducting layer (cathode part) from the anode part.

The masking material which can be used is a general heat-resistantresin, preferably a heat-resistant resin soluble in or swellable with asolvent, a precursor thereof or a composition comprising an inorganicfine powder and a cellulose-base resin (see, JP-A-11-80596).

Examples thereof include polyphenylsulfone(PPS), polyethersulfone(PES),cyanic acid ester resin, fluoro-resin (tetrafluoroethylene,tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer and the like),low molecular weight polyimide and derivatives thereof. Among these,preferred are polyimide having a low molecular weight, polyethersulfone,fluororesin and precursors thereof, and more preferred is polyimidehaving a low molecular weight.

Such a material is linearly coated as a solution or dispersion solutionof an organic solvent, thermally deformed to form a polymer by heattreatment and then cured.

The masking may be performed by a method of attaching a tape made ofpolypropylene, polyester, silicon-base resin, fluorine-base resin or thelike, or a method of forming a resin coat film part.

The masking is applied to the boundary part between theanode-leading-out-part 2 and the region 3 where a solid electrolyte 7 isformed.

(6) Formation of Solid Electrolyte

The electrically conducting polymer for forming a solid electrolyte usedin the solid electrolytic capacitor of the present invention is notlimited but an electrically conducting polymer having a π electronconjugate structure is preferably used and examples thereof includeelectrically conducting polymers containing, as a repeating unit, astructure shown by a compound having a thiophene skeleton, a compoundhaving a polycyclic sulfide skeleton, a compound having a pyrroleskeleton, a compound having a furan skeleton or a compound having ananiline skeleton.

Among the monomers used as a starting material of the electricallyconducting polymer, examples of the compound having a thiophene skeletoninclude a compound represented by formula (I):

(wherein the substituents R¹ and R² each independently represents amonovalent group selected from the group consisting of a hydrogen atom,a linear or branched, saturated or unsaturated hydrocarbon group havinga carbon number of 1 to 10, an alkoxy group, an alkyl ester group, ahalogen, a nitro group, a cyano group, a primary, secondary or tertiaryamino group, a CF₃ group, a phenyl group and a substituted phenyl group,the hydrocarbon chains of R¹ and R² may combine with each other at anarbitrary position to form a divalent chain for forming at least one 3-,4-, 5-, 6- or 7-membered saturated or unsaturated hydrocarbon ringstructure together with the carbon atoms substituted by the groups R¹and R², and the combined ring chain may arbitrarily contain a bond ofcarbonyl, ether, ester, amide, sulfide, sulfinyl, sulfonyl or imino).

Specific examples thereof include derivatives such as 3-methylthiophene,3-ethylthiophene, 3-propylthiophene, 3-butylthiophene,3-pentylthiophene, 3-hexylthiophene, 3-heptylthiophene,3-octylthiophene, 3-nonylthiophene, 3-decylthiophene, 3-fluorothiophene,3-chlorothiophene, 3-bromothiophene, 3-cyanothiophene,3,4-dimethylthiophene, 3,4-diethylthiophene, 3,4-butylenethiophene,3,4-methylenedioxythiophene and 3,4-ethylenedioxythiophene. Thesecompounds may be a compound available on the market or may be preparedby a known method (a method described, for example, in Synthetic Metals,Vol. 15, page 169 (1986)).

Specific examples of the compound having a polycyclic sulfide skeletoninclude compounds having a 1,3-dihydro-polycyclic sulfide (also called1,3-dihydrobenzo-[c]thiophene) skeleton and compounds having a1,3-dihydronaphtho[2,3-c]thiophene skeleton. Furthermore, compoundshaving a 1,3-dihydroanthra[2,3-c]thiophene skeleton and compounds havinga 1,3-dihydronaphthaceno[2,3-c]thiophene skeleton may be used. Thesecompounds may be prepared by a known method, for example, the methoddescribed in JP-A-8-3156.

In addition, for example, compounds having a1,3-dihydronaphtho[1,2-c]thiophene skeleton,1,3-dihydrophenanthra[2,3-c]thiophene derivatives, compounds having a1,3-dihydrotriphenylo[2,3-c]thiophene skeleton and1,3-dihydrobenzo[a]anthraceno[7,8-c]thiophene derivatives may also beused.

A compound arbitrarily containing nitrogen or N-oxide in the condensedring may also be used and examples thereof include1,3-dihydrothieno[3,4-b]quinoxaline,1,3-dihydrothieno[3,4-b]quinoxaline-4-oxide and1,3-dihydrothieno[3,4-b]quinoxaline-4,9-dioxide, however, the presentinvention is not limited thereto.

Specific examples of the compound having a pyrrole skeleton includederivatives such as 3-methylpyrrole, 3-ethylpyrrole, 3-propylpyrrole,3-butylpyrrole, 3-pentylpyrrole, 3-hexylpyrrole, 3-heptylpyrrole,3-octylpyrole, 3-nonylpyrrole, 3-decylpyrrole, 3-fluoropyrrole,3-chloropyrrole, 3-bromopyrrole, 3-cyanopyrrole, 3,4-dimethylpyrrole,3,4-diethylpyrrole, 3,4-butylenepyrrole, 3,4-methylenedioxypyrrole and3,4-ethylenedioxypyrrole, however, the present invention is not limitedthereto. These compounds may be a commercially available compound or maybe prepared by a known method.

Specific examples of the compound having a furan skeleton includederivatives such as 3-methylfuran, 3 ethylfuran, 3-propylfuran,3-butylfuran, 3-pentylfuran, 3-hexylfuran, 3-heptylfuran, 3-octylfuran,3-nonylfuran, 3-decylfuran, 3-fluorofuran, 3-chlorofuran, 3-bromofuran,3-cyanofuran, 3,4-dimethylfuran, 3,4-diethylfuran, 3,4-butylenefuran,3,4-methylenedioxyfuran and 3,4-ethylenedioxyfuran, however, the presentinvention is not limited thereto. These compounds may be a commerciallyavailable compound or may be prepared by a known method.

Specific examples of the compound having an aniline skeleton includederivatives such as 2-methylaniline, 2-ethylaniline, 2-propylaniline,2-butylaniline, 2-pentylaniline, 2-hexylaniline, 2-heptylaniline,2-octylaniline, 2-nonylanilin, 2-decylaniline, 2-fluoroaniline,2-chloroaniline, 2-bromoaniline, 2-cyanoaniline, 2,5-dimethylaniline,2,5-diethylaniline, 2,3-butyleneaniline, 2,3-methylenedioxyaniline and2,3-ethylenedioxyaniline, however, the present invention is not limitedthereto. These compounds may be a commercially available product or maybe prepared by a known method.

The compounds selected from the group consisting of the above-describedcompounds may also be used in combination to form the solid electrolyteas a copolymer. In this case, the composition ratio and the like ofpolymerizable monomers vary depending on the polymerization conditionsand the like, but preferred composition ratio and polymerizationconditions can be confirmed by a simple test. Examples of the methodwhich can be used therefor include a method where a monomer and anoxidizing agent each preferably in the form of a solution are coatedseparately one after another or coated simultaneously on an oxide filmlayer of a metal foil to form a solid electrolyte (see, JP-A-2-15611 andJP-A-10-32145 (U.S. Pat. No. 6,214,930)). Generally, in the electricallyconducting polymer, an arylsulfonic acid-base dopant such as salts ofbenzenesulfonic acid, toluenesulfonic acid, naphthalenesulfonic acid,anthracenesulfonic acid or anthraquionenesulfonic acid can be used as adopant-donating agent.

As shown in FIG. 4, a carbon paste layer and a metal powder-containingelectrically conducting layer (not shown) are provided on the surface ofthe solid electrolyte layer 7 to form the cathode part 8 of a capacitor.The metal powder-containing electrically conducting layer is tightlyjoined with the solid electrolyte layer and acts not only as the cathodebut also as an adhesive layer for joining a cathode lead terminal 9 of afinal capacitor product (FIG. 5). The thickness of the metalpowder-containing electrically conducting layer is not limited but thethickness is generally on the order of 10 to 100 μm, preferably on theorder of 10 to 50 μm.

In the case of composing a multilayer solid electrolytic capacitor byusing two or more capacitor elements of the present invention, as oneembodiment specifically shown in FIG. 5, a plurality of stackedcapacitor elements are welded at anode-leading-out-parts, and a leadframe 11 is jointed to the edge surface of the welded part. The leadframe 11 may be chamfered, that is, may have a shape where edge cornerparts are shaved and thereby slightly flattened or rounded.

Furthermore, the cathode bonding part and the anode bonding partopposing the lead frame may be rendered to undertake the roles of leadterminals 9 and 13.

The material for the lead frame is not particularly limited if it is amaterial generally used, but the lead frame is preferably constituted bya copper-base (for example, Cu—Ni, Cu—Ag, Cu—Sn, Cu—Fe, Cu—Ni—Ag,Cu—Ni—Sn, Cu—Co—P, Cu—Zn—Mg or Cu—Sn—Ni—P alloy) material or a materialwith the surface being plated with a copper-base material and whenconstituted as such, this provides advantages of, for example, reducingthe resistance by devising the shape of the lead frame and obtaininggood workability for chamfering of lead frame.

As shown in the cross-sectional view of FIG. 5, a lead terminal 13 isbonded to the lead frame 11 joined to the anode part 12, a lead terminal9 is bonded to the cathode part 8 comprising a solid electrolyte layer7, a carbon paste layer and a metal powder-containing electricallyconducting layer, and the whole is molded with an insulating resin 15such as epoxy resin, whereby a solid electrolytic capacitor 14 isobtained.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described in greater detail by referring torepresentative examples. These are mere examples for the purpose ofexplanation and the present invention is not limited thereto by anymeans.

EXAMPLE 1

Step of Making Cut Lines

On a 200 μm-thick aluminum foil (containing Si: 20 ppm by mass, Fe: 24ppm by mass, Cu: 33 ppm by mass and Ti: 0.9 ppm by mass), cut lines of arectangular shape with one side open, each having a width of 200 μm wereformed. Each of the rectangular shaped in cut lines to form a capacitorelement had a width of 3 mm and a length of 6 mm. As shown in FIG. 1(A),cut lines for 30 capacitor elements were arranged in 3 lines×10 rows.

Etching Step

Both front and back surfaces of the portion working out to ananode-leading-out-part was covered with a 1 mm-wide resin tape asprotective material (FIG. 1(B)) and then, the aluminum foil was dippedin a first electrolytic solution (10 mass % of hydrochloric acid +0.5mass % of aqueous sulfuric acid solution) at 60° C. and etched by ACelectrolytic etching under the conditions shown in Table 1.

Electrochemical Formation Step

The resin tape was removed (FIG. 1(C)) and a strip cut out in acomb-like shape from the aluminum foil by a cutter (FIG. 1(D)) wasdipped in an aqueous ammonium adipate solution and applied with avoltage of 13 V to electrochemically form the electrochemicallynon-formed part and thereby form a dielectric film.

Masking Step

Along the anode-leading-out-part side, masking 4 b with a 0.5-mm resintape was applied to a portion 5 mm distant from the end of the portionwhere a solid electrolyte was to be formed on, in order to control theregion for solid electrolyte 7, carbon paste and silver paste to beformed on.

Solid Electrolyte Formation Step

A solid electrolyte was formed as follows in the electrochemicallyformed layer region.

The capacitor element tips of the aluminum foil strips were dipped in anisopropanol solution containing 20 mass % of 3,4-ethylenedioxythiophene(Solution 1), then pulled out and left stand at 25° C. for 5 minutes.Thereafter, the aluminum foil in the portion treated with the monomersolution was dipped in an aqueous solution containing 30 mass % of anaqueous ammonium persulfate solution prepared to have a sodium2-anthraquinonesulfonate (produced by Tokyo Kasei) concentration of 0.07mass %, and then dried at 60° C. for 10 minutes, thereby performing theoxidative polymerization. The operation from dipping in Solution 1 untildipping in Solution 2 to perform the oxidative polymerization wasrepeated 25 times and thereby a solid electrolyte layer was formed. Onthis solid electrolyte layer, a carbon paste and a silver paste werecoated. The aluminum foil was cut out from the aluminum foil strip, as aresult, a solid electrolyte capacitor element 8 shown in FIG. 4 wasobtained.

Fabrication and Test of Chip-Type Multilayer Solid ElectrolyticCapacitor

Two solid electrolytic capacitor elements were stacked by joining theseon a lead frame using a silver paste, an anode lead terminal wasconnected by welding to the portion where an electrically conductingpolymer was not formed, the whole was molded with epoxy resin, and theobtained capacitor element was aged for 2 hours by applying a ratedvoltage (6.3 V) at 120° C. In this way, 150 units in total of chip-typesolid electrolytic capacitors were manufactured.

The obtained multilayer solid electrolytic capacitor were subjected to areflow test by passing each capacitor through a region at a temperatureof 230° C. for 30 minutes, the leakage current 1 minute after theapplication of rated voltage was measured, an average leakage current(μA) of those having a measured value of 1 CV or less at a rated voltage(6.3V) was determined, those having a measured value of 0.04 CV or morewere evaluated as leakage current defective, those having a capacitance30% or more lower than the capacitance value of a capacitor estimatedfrom the capacitance measured by dipping a capacitor element in ammoniumadipate solution (15%) after electrochemical formation were evaluated ascapacitance defective, those evaluated as capacitance defective weredisassembled and inspected, those having disengagement of the anodeelectricity taking out portion from the lead were evaluated as weldingdefective, and the “number of defective units/number of units evaluated”was determined. The results obtained are shown in Table 2.

With respect to r and T2/T1, the values were obtained through actualmeasurements on optical micrographs after polishing the cut surface ofthe solid electrolytic capacitor obtained. In a case where a foil afteretching treatment is cut out through punching in a shape of a capacitorelement, since the cut edge surface is almost perpendicular to the flatsurface of the foil and the cut edge surface where the core metal of thefoil is exposed does not form an etching layer, the values r and T2 areboth 0.

EXAMPLE 2

Multilayer solid electrolytic capacitors were fabricated in the samemanner as in Example 1 except for changing the thickness of aluminumfoil from 200 μm to 300 μm. The measurement of leakage current and thereflow test were performed in the same manner. The results obtained areshown in Table 2.

EXAMPLE 3

Capacitors were completed in the same manner as in Example 1 except thatin the etching step, the portion working out to theanode-leading-out-part was not protected by the protective materialresin tape in Example 1. These capacitor elements were evaluated on theproperties in the same manner as in Example 1. The results obtained areshown in Table 2.

EXAMPLE 4

Capacitors were completed in the same manner as in Example 1 except forusing pyrrole in place of 3,4-ethylenedioxythiophene in Example 1. Thesecapacitor elements were evaluated on the properties in the same manneras in Example 1. The results obtained are shown in Table 2.

EXAMPLE 5

Capacitors were completed in the same manner as in Example 1 except forusing furan in place of 3,4-ethylenedioxythiophene in Example 1. Thesecapacitor elements were evaluated on the properties in the same manneras in Example 1. The results obtained are shown in Table 2.

EXAMPLE 6

Capacitors were completed in the same manner as in Example 1 except forusing the etching current having a triangular waveform in place of theetching current having a sine waveform. These capacitor elements wereevaluated on the properties in the same manner as in Example 1. Theresults obtained are shown in Table 2.

EXAMPLE 7

Capacitors were completed in the same manner as in Example 1 except forusing the etching current having a rectangular waveform in place of theetching current having a sine waveform. These capacitor elements wereevaluated on the properties in the same manner as in Example 1. Theresults obtained are shown in Table 2.

COMPARATIVE EXAMPLE 1

Multilayer solid electrolytic capacitors were fabricated in the samemanner as in Example 1 except using an aluminum foil having a thicknessof 100 μm, etching the foil having no cut lines and cutting the etchedfoil into pieces having a predetermined size in place of steps of makingcut lines and then etching in Example 1. The measurement of leakagecurrent and the reflow test were performed in the same manner. Theresults obtained are shown in Table 2.

COMPARATIVE EXAMPLE 2

Multilayer solid electrolytic capacitors were fabricated in the samemanner as in Example 1 except for making cuts in the aluminum foil suchthat the acute angle A is 20°. The measurement of leakage current andthe reflow test were performed in the same manner. The results obtainedare shown in Table 2.

COMPARATIVE EXAMPLE 3

Multilayer solid electrolytic capacitors were fabricated in the samemanner as in Example 1 except for making cuts in the aluminum foil suchthat the width of the cut line is 3 mm. The measurement of leakagecurrent and the reflow test were performed in the same manner. Theresults obtained are shown in Table 2. TABLE 1 Current Quantity of AcuteFrequency Density Electricity Width Angle A Waveform (Hz) (A/cm²)(C/cm²) (mm) (degree) Ex. 1 sine 30 0.5 400 0.2 90 Ex. 2 sine 30 0.5 6500.2 90 Ex. 3 sine 30 0.5 400 0.2 90 Ex. 4 sine 30 0.5 400 0.2 90 Ex. 5sine 30 0.5 400 0.2 90 Ex. 6 triangular 30 0.5 400 0.2 90 Ex. 7rectangular 30 0.5 400 0.2 90 Compar. Ex. 1 — — — — — 90 Compar. Ex. 2sine 30 0.5 400 0.2 20 Compar. Ex. 3 sine 30 0.5 400 3.0 90

TABLE 2 Leakage Average Average Capacitance Welding Current Leakage rT2/ Capacitance Defective Defective Defective Current (μm) T1 (μF)Ratio* Ratio* Ratio* (μA) Ex. 1 52 0.7 63.5 0/150 0/150 0/150 0.15 Ex. 252 0.7 93.2 0/150 0/150 0/150 0.18 Ex. 3 52 0.7 62.2 3/150 3/150 2/1500.17 Ex. 4 52 0.7 63.6 0/150 0/150 0/150 0.15 Ex. 5 52 0.7 63.5 0/1500/150 0/150 0.15 Ex. 6 48 0.6 61.0 0/150 0/150 0/150 0.10 Ex. 7 47 0.662.5 0/150 0/150 0/150 0.14 Compar. Ex. 1 0 0.0 27.1 5/150 5/150 22/150 1.89 Compar. Ex. 2 10 0.3 59.0 0/150 0/150 5/150 0.78 Compar. Ex. 3 892.8 50.7 1/150 0/150 10/150  0.95*number of defective units/number of units evaluated

INDUSTRIAL APPLICABILITY

According to the present invention, the following effects are obtained.

(1) By making cut lines in a partial shape of a capacitor element beforeetching process, a valve-acting metal foil for capacitor elements,uniform in effective area, can be obtained, so that porous valve-actingmetal, with narrow variation in capacitance, may be prepared.

A porous layer is formed also on the cut edge surface of a porous valveacting metal in the portion where at least an electrically conductingpolymer is formed, and the sharp-edged corner part of the cut partdissolves by etching and forms a curved face, a high capacitorcapacitance can be obtained and the generation of defectives due toincrease in the leakage current after molding and reflow can beprevented.

(2) A porous layer being not formed on the anode-leading-out-part, noelectrically conducting polymer is formed on the anode-leading-out-partby a capillary phenomenon in the chemical polymerization, therefore,short circuit due to the formation of an electrically conducting polymerdoes not occur and the welding at the stacking of elements isfacilitated, as a result, defectives due to welding failure decrease,the contact resistance becomes small and a capacitor having a smallequivalent series resistance can be obtained.

1. A method for producing a metal foil for capacitors, comprising a stepof making cut lines in a valve-acting metal foil in a shape of acapacitor element with at least a part of a portion predetermined to beanode-leading-out-part left uncut, a step of etching the cut edgesurface generated in the previous step and the surface part of thevalve-acting metal foil, and a step of electrochemically forming themetal foil.
 2. The method for producing a metal foil for capacitors asclaimed in claim 1, wherein the etching is performed after protectingthe portion predetermined to be the anode-leading-out-part of acapacitor element with a protective material.
 3. The method forproducing a metal foil for capacitors as claimed in claim 2, wherein theprotective material is removed after etching the valve-acting metalfoil, and then the step of electrochemically forming is performed. 4.The method for producing a metal foil for capacitors as claimed in claim2, wherein the protective material is removed after electrochemicallyforming the etched foil.
 5. The method for producing a metal foil forcapacitors as claimed in claim 2, wherein the protective material isremoved after etching, and masking is applied to the boundary betweenthe anode-leading-out-part and the region to have a solid electrolyticlayer formed thereon to serve as a cathode part before performing thestep of electrochemically forming the region to be a cathode part. 6.The method for producing a metal foil for capacitors as claimed in claim1, wherein each of the cut portions has a quadrangular-shape havinguncut portion, U-shape(horseshoe-shape) or semicircular shape;
 7. Themethod for producing a metal foil for capacitors as claimed in claim 1,wherein the cut edge surface has an acute interior angle A of 30° ormore with respect to the metal foil surface.
 8. The method for producinga metal foil for capacitors as claimed in claim 1, wherein the width dof the cut line is twice or less the thickness of the metal foil.
 9. Themethod for producing a metal foil for capacitors as claimed in claim 1,wherein a plurality of capacitors is produced in a single batch processby making plurality of cut lines each having a shape of a capacitorelement in a single valve-acting metal foil.
 10. The method forproducing a metal foil for capacitors as claimed in claim 1, wherein thefoil consists of at least one valve-acting metal selected from a groupof aluminum, niobium and tantalum.
 11. The method for producing a metalfoil for capacitors as claimed in claim 1, wherein the valve-actingmetal foil has a thickness of 0.05 to 1 mm.
 12. The method for producinga metal foil for capacitors as claimed in claim 1, wherein thevalve-acting metal foil is an aluminum foil containing at least oneelement selected from the group consisting of Si, Fe, Cu, Zn, Ni, Mn,Ti, Pb, B, P, V and Zr.
 13. The method for producing a metal foil forcapacitors as claimed in claim 12, wherein the total content of theelements other than aluminum contained in the foil is from 1 to 1,000ppm by mass.
 14. The method for producing a metal foil for capacitors asclaimed in claim 12, wherein the aluminum foil contains Si in an amountof 1 to 100 ppm by mass, Fe in an amount of 1 to 100 ppm by mass and Cuin an amount of 1 to 100 ppm by mass.
 15. The method for producing ametal foil for capacitors as claimed in claim 1, wherein the etching isAC electrolytic etching using at least one waveform selected from thegroup consisting of sine wave, rectangular wave and triangular wave. 16.The method for producing a metal foil for capacitors as claimed in claim1, wherein the etching is AC electrolytic etching where terminals areprovided on the valve-acting metal and on electrodes placed to bothsides of the valve-acting metal and AC current is directly supplied tothe terminal provided on the valve-acting metal.
 17. The method forproducing a metal foil for capacitors as claimed in claim 1, wherein theetching is DC electrolytic etching.
 18. A metal foil for capacitors,obtained by the production method according to claim
 1. 19. The metalfoil for capacitors as claimed in claim 18, wherein the edge of the cutportion has a curvature radius r of 0.1 to 500 μm.
 20. The metal foilfor capacitors as claimed in claim 18 above, comprising, on the surfaceof the metal foil and the cut edge surface, porous layers formed on aportion where solid electrolyte is to be formed, wherein the thicknessof the porous layer on the cut edge surface, T2, is more than 1 μm, andhas a following relationship with the thickness of the porous layer onthe surface of the metal foil, T1:T 2/T 1≦2
 21. A solid electrolytic capacitor element, comprising a solidelectrolyte layer and an electrically conducting layer in the order onthe metal foil according to claim
 18. 22. The solid electrolyticcapacitor element as claimed in claim 21, wherein the solid electrolytelayer comprises an electrically conducting polymer.
 23. The solidelectrolytic capacitor element as claimed in claim 22, wherein a monomerforming the electrically conducting polymer is a monomer compoundcontaining a heterocyclic 5-membered ring or a monomer compound havingan aniline skeleton.
 24. The solid electrolytic capacitor element asclaimed in claim 23, wherein the monomer compound containing aheterocyclic 5-membered ring is a compound selected from the groupconsisting of pyrrole, thiophene, furan, polycyclic sulfide andsubstitution derivatives thereof.
 25. The solid electrolytic capacitorelement as claimed in claim 23, wherein the monomer compound containinga heterocyclic 5-membered ring is a compound represented by thefollowing formula (I):

wherein the substituents R¹ and R² each independently represents amonovalent group selected from the group consisting of a hydrogen atom,a linear or branched, saturated or unsaturated hydrocarbon group havinga carbon number of 1 to 10, an alkoxy group, an alkyl ester group, ahalogen, a nitro group, a cyano group, a primary, secondary or tertiaryamino group, a CF₃ group, a phenyl group and a substituted phenyl group,the hydrocarbon chains of R¹ and R² may combine with each other at anarbitrary position to form a divalent chain for forming at least one 3-,4-, 5-, 6- or 7-membered saturated or unsaturated hydrocarbon ringstructure together with the carbon atoms substituted by the groups R¹and R², and the combined ring chain may arbitrarily contain a bond ofcarbonyl, ether, ester, amide, sulfide, sulfinyl, sulfonyl or imino. 26.The solid electrolytic capacitor element as claimed in claim 23, whereinthe monomer compound containing a heterocyclic 5-membered ring is acompound selected from 3,4-ethylenedioxythiophene and1,3-dihydroisothianaphthene.
 27. A multilayer solid electrolyticcapacitor obtained by stacking a plurality of capacitor elementsaccording to claim
 21. 28. A method for producing solid electrolyticcapacitor elements, comprising a step of making cut lines each having ashape of a capacitor element with at least a part of a portionpredetermined to be anode-leading-out-part left uncut in a valve-actingmetal foil, a step of etching the cut edge surface generated in theprevious step and the surface of the valve-acting metal foil, a step ofelectrochemically forming the etched metal foil to form an oxidedielectric film after cutting the foil into stripes each having acomb-like shape where foil portions each cut in a shape of an elementlink together in anode-leading-out-parts, a step of forming a solidelectrolyte layer on the oxide dielectric film layer, a step of formingan electrically conducting layer on the solid electrolyte layer, and astep of severing the foil pieces each in a shape of a capacitor elementby making a cut in the anode-leading-out-part of each piece.